1. Field of the Invention
The present invention relates to a driver circuit formed on an insulation surface. Further, it also relates to a display device having the driver circuit and a light emitting element provided on the insulation surface. Particularly, the present invention relates to an active matrix display having the driver circuit and a plurality of pixels which are arranged in matrix, and a switching element and a light emitting element are arranged in each pixel.
2. Description of the Related Art
An active matrix display device having a plurality of pixels in which a switching element and a light emitting element are arranged in each pixel, has advantages like superiority in response, operation with a low voltage and a wide view angle. Therefore, the active matrix display device comes under the spotlight as a next generation flat panel display.
Incidentally, the light emitting element means an element of which a luminance is controlled by electric current or voltage. For the light emitting element, electron source elements typified by an OLED (organic light emitting diode) element, a FE (field emission display) element, an MIM (Metal-Insulator-Metal) element and the like may be used.
The light emitting element comprises of an anode, a cathode, and a layer containing an organic compound (hereafter simply referred to as an organic compound layer) and sandwiched between the anode and the cathode. The light emitting element emits light according to a voltage applied between the anode and the cathode. Note that emitting the light emitting element is referred to as driving the light emitting element.
An organic compound layer usually has a lamination structure. A typical lamination structure thereof is one proposed by Tang et al. of Eastman Kodak Company and consisting of a hole-transporting layer, a light emitting layer, and an electron transporting layer. Other examples of the lamination structure include one in which a hole injection layer, a hole transporting layer, a light emitting layer, and an electron transporting layer are layered in order on an anode, and one in which a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injection layer are layered in order on an anode. A light emitting layer may be doped with a fluorescent pigment or the like. A given voltage is applied to the organic compound layer structured as above from a pair of electrodes (an anode and a cathode) to induce recombination of carriers in its light emitting layer. As a result, the light emitting layer emits light.
At this time, the emission luminance of the light emitting element is in proportion to the current flowing between the electrodes (the anode and the cathode). Accordingly, a pixel structure in which a current flowing to the light emitting element of each pixel is controlled by a current (hereafter referred to as control current) input to the pixel portion, is proposed. Such a pixel structure is referred as to current control type pixel.
An example of the structure of the current control type pixel in the active matrix display device is shown with FIG. 7.
As shown in FIG. 7, the current control type pixel comprises of a signal line 701, a scanning line 702, a power source line 703, a wiring 710, a light emitting element 709, a switching transistor 704, a current storage transistor 705, a current transistor 706 consisting of a current mirror circuit, a driver transistor 707 consisting of a current mirror circuit and using for driving the light emitting element, and a storage capacitor 708.
Either of a source electrode and a drain electrode of the switching transistor 704 is connected to the signal line 701, and the other is connected to a drain of the current transistor 706 and either of a source electrode and a drain electrode of the current storage transistor 705, further, the gate electrode of the switching transistor 704 is connected to the scanning line 702.
A source electrode of the current transistor 706 is connected to the power source line 703. A source electrode of the current storage transistor 705 or one side of a drain electrode of the current storage transistor 705 disconnected with the switching transistor 704 is connected to one of electrodes of the storage capacitor 708, a gate electrode of the current transistor 706 and a gate electrode of the driver transistor 707.
A side of the storage capacitor 708 disconnected with the current storage transistor 705 is connected to the power source line 703. A source electrode of the driver transistor 707 is connected to the power source line 703, while a drain electrode of the driver transistor 707 is connected to one of electrodes of the light emitting element 709.
Next, a driving method (operation method) in which a video signal is input to the pixel shown in FIG. 7 and the light emitting element emits light, will be described as below. Note that as the video signal input to the pixel, a current (signal current) having a current magnitude corresponding to the luminance represented by the pixel is input. In the pixel shown in FIG. 7, the control current which controls the current flowing to the light emitting element in each pixel is identical with the video signal (signal current).
A signal is input to the scanning line 702, thereby the switching transistor 704 is in state of ON, then, the signal current input from the signal line 701 is input to the pixel. At this time, the current storage transistor 705 is in state of conductive by the signal input to the wiring 710.
After the signal current is input to the pixel, as time sufficiently goes by, the signal current turns to flow between the source and the drain of the current transistor 706. At this time, in the storage capacitor 708, a gate voltage (a voltage between the gate and the source), in order that the current transistor 706 flows a signal current as a drain current, is retained. The signal of the wiring 710 changes thereafter, the current storage transistor 705 becomes in a non-conductive state.
In case that the characteristics of the current transistor 706 and driver transistor 707 are uniform, the drain current of the current transistor 706 is equal to the drain current of the driver transistor 707. At this time, a current equal to the signal current input from the power source line 703 thorough the driver transistor 707 is input to the light emitting element 709. After this manner, the light emitting element 709 emits light in a luminance corresponding to the signal current.
Note that even after the signal current is not input to the pixel, the driver transistor 707 flows a current equal to the signal current by the voltage retained in the storage capacitor 708.
FIG. 8 is a block diagram showing the configuration of an active matrix display device having the current control pixel illustrated in FIG. 7.
FIG. 8 illustrates a pixel portion 804, scanning driver circuits 803a, 803b which input signals to the scanning line of each pixel in the pixel portion 804, and a signal line driver circuit 802 which input signals to the signal line of each pixel in the pixel portion 804. The pixel portion. 804 and the scanning driver circuits 803a, 803b are provided on a substrate (hereafter referred to as pixel substrate) 801 having an insulation surface. The signal line driver circuit 802 which are out of a LSI chip 806 and the like, and the LSI chip 806 is attached to the pixel substrate 801 by a TAB 805.
Note that in the current control pixel shown in FIG. 7, the driver circuit input control current is denoted as a control current output circuit. In the configuration of the display device shown in FIG. 8, the control current output circuit corresponds to the signal line driver circuit.
Further, a wiring providing control current output from the control current output circuit to the pixel portion is denoted as a control current line. In the pixel portion shown in FIG. 7, the control current line corresponds to the signal line 701.
As shown in FIG. 8, the driver circuit (control current output circuit) inputting control current to the current control type pixel portion is formed from the LSI chip on a monocrystal substrate. The monocrystal substrate, in which the control current output driver circuit is formed, is attached to the pixel substrate by using the TAB and the like. In this way, the pixel portion can be electrically connected to the control current output circuit.
However, an area of overlap width is required when the control current output circuit is attached. Further, the wiring resistance and wiring capacitance between the control current output circuit and the pixel portion which are electrically connected with each other become large so that a low power consumption display device can't be realized.
Therefore, it is desirable to use a polycrystalline transistor to form the control current output circuit on the pixel substrate. Further, it becomes possible to set driver frequency highly by forming the control current output circuit with a polycrystalline transistor.
On the other hand, for the control current output circuit formed by using a polycrystalline, there is a problem with wide dispersion of output current due to influence of crystalline dispersion of channel formation region, etc. As described above, the light emitting element emits light in a luminance commensurate with the flowing current. Therefore, when dispersion exits among the pixels, a dispersion of luminance of light emitting element in pixel (hereafter also referred to as unevenness of display) is caused.